Light-emitting element, light-emitting device, electronic device, and lighting device

ABSTRACT

Light-emitting elements in which an increase of driving voltage can be suppressed are provided. Light-emitting devices whose power consumption is reduced by including such light-emitting elements are also provided. In a light-emitting element having an EL layer between an anode and a cathode, a first layer in which carriers can be produced is formed between the cathode and the EL layer and in contact with the cathode, a second layer which transfers electrons produced in the first layer is formed in contact with the first layer, and a third layer which injects the electrons received from the second layer into the EL layer is formed in contact with the second layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting element in which anorganic compound is provided between a pair of electrodes and theorganic compound exhibits luminescence by application of electric field.Further, the present invention relates to a light-emitting device havingsuch a light-emitting element. Furthermore, the present inventionrelates to an electronic device and a lighting device having such alight-emitting device.

2. Description of the Related Art

Light-emitting elements, which use organic compounds as light emittersand have features of thinness, lightweight, fast response, and directcurrent low voltage driving, are expected to be applied tonext-generation flat panel displays. In particular, display deviceshaving light-emitting elements arranged in matrix are considered to besuperior to conventional liquid crystal display devices, because theyhave wide viewing angle and excellent visibility.

A light emission mechanism of a light-emitting element is describedbelow: when voltage is applied to a pair of electrodes with an EL layerincluding a light emitter interposed therebetween, electrons injectedfrom a cathode and holes injected from an anode are recombined at anemission center in the EL layer to form molecular exciton, and energy isreleased when the molecular exciton relaxes to the ground state and thuslight is emitted. An excited singlet state and an excited triplet stateare known as an excited state, and it is thought that light can beemitted through either state.

As for such a light-emitting element, improvement of an elementstructure, a development of a material, and the like have been conductedin order to improve element characteristics.

For example, it is reported that an organic compound included in anelectron-injecting layer formed in contact with a cathode, is doped witha metal having a low work function, such as an alkali metal, an alkalineearth metal or a rare earth metal, so that injection barrier can bereduced in injection of electrons from the cathode into theelectron-injecting layer including an organic compound and thus drivingvoltage can be reduced (e.g., see Patent Document 1).

Further, it is also reported that an optical adjustment of an emissionspectrum can be conducted without increase of a driving voltage, inrelation to the above-described technology (e.g., see Patent Document2).

Specifically, between a cathode and an EL layer in a light-emittingelement, a layer in which a hole-transporting organic compound is dopedwith a metal oxide is formed in contact with the cathode, and a layer inwhich an electron-transporting organic compound is doped with a metalhaving a low work function, such as an alkali metal, an alkaline earthmetal, or a rare earth metal, is formed so as to be in contact with thelayer doped with metal oxide, and the thickness of the layer doped withmetal oxide is increased to conduct an optical adjustment of an emissionspectrum. In this case, because the hole-transporting organic compoundhas higher carrier mobility than the electron-transporting organiccompound, an increase of driving voltage can be more suppressed than ina case where the thickness of the layer in which theelectron-transporting organic compound is doped with a metal having lowwork function is increased.

[Reference]

[Patent Documents]

-   [Patent Document 1] Japanese Published Patent Application No.    H10-270171-   [Patent Document 2] Japanese Published Patent Application No.    2005-209643

SUMMARY OF THE INVENTION

However, in the structure as in Patent Document 2 in which a layerincluding an acceptor substance and a layer including a donor substanceare in contact with each other, influence on spatial structure(production of p-n junction leads to formation of a depletion layer dueto carrier movement) or interactive functional interference between theacceptor substance and the donor substance causes an increase of drivingvoltage unfortunately.

In view of the above-described problem, it is an object of oneembodiment of the present invention to provide light-emitting elementsin which an increase of diving voltage can be suppressed. In addition,it is another object to provide light-emitting devices whose powerconsumption is reduced by including such light-emitting elements.

It is an object of one embodiment of the present invention to providelight-emitting elements in which an increase of driving voltage can besuppressed even when a thickness of a layer provided between electrodesof such light-emitting elements is changed. In addition, it is anotherobject to provide light-emitting devices whose power consumption isreduced by including such light-emitting elements.

It is an object of one embodiment of the present invention to providelight-emitting elements in which an increase of driving voltage can besuppressed even when a thickness of a layer provided between electrodesof such light-emitting elements is changed, and of which opticaladjustment can be conducted. In addition, it is another object toprovide light-emitting devices whose power consumption is reduced andwhich can have excellent color purity by including such light-emittingelements.

It is an object of one embodiment of the present invention to suppressan increase of driving voltage for light-emitting elements each having alayer including an acceptor substance and a layer including a donorsubstance. In addition, it is another object to provide light-emittingdevices whose power consumption is reduced by including suchlight-emitting elements.

It is an object of one embodiment of the present invention to providelight-emitting elements configured so that an acceptor substance in alayer including the acceptor substance and a donor substance in a layerincluding the donor substance hardly interact with each other and hardlyinterfere with their functions mutually. In addition, it is anotherobject to provide light-emitting devices whose power consumption isreduced by including such light-emitting elements.

In addition, according to one embodiment of the present invention, in alight-emitting element having an EL layer between an anode and acathode, a first layer in which carriers can be produced is formedbetween the cathode and the EL layer and in contact with the cathode, asecond layer which transfers (donates and accepts) electrons produced inthe first layer is formed in contact with the first layer, and a thirdlayer which injects the electrons received from the second layer intothe EL layer is formed in contact with the second layer.

Note that the first layer is formed so as to include a highhole-transporting substance and an acceptor substance, and holes of thecarriers produced in the first layer move into the cathode whileelectrons move into the second layer.

In addition, as the substance included in the second layer, a highelectron-transporting layer having a slightly higher LUMO level(preferably, −5.0 eV or higher, more preferably from −5.0 eV to −3.0 eV)than an acceptor level of an acceptor substance included in the firstlayer is used, so that electron transfer can be easily conducted fromthe first layer to the second layer.

In addition, the third layer is formed using a high electron-injectingsubstance such as an alkali metal, an alkaline earth metal, a rare earthmetal, an alkali metal compound, an alkaline earth metal compound, or arare earth metal compound, or a high electron-transporting substanceincluding a donor substance, and thereby injection barrier in injectionof electrons into the EL layer can be reduced.

One embodiment of the present invention is a light-emitting elementincluding at least an EL layer between an anode and a cathode; a firstlayer including a high hole-transporting substance and an acceptorsubstance, provided in contact with the cathode between the cathode andthe EL layer; a second layer including a high electron-transportingsubstance, provided in contact with the first layer; and a third layerincluding at least one selected from the group consisting of an alkalimetal, an alkaline earth metal, a rare earth metal, an alkali metalcompound, an alkaline earth metal compound, and a rare earth metalcompound, provided in contact with the second layer and the EL layer.

In addition, in the above structure, the EL layer may include a fourthlayer including a high electron-transporting substance, and the fourthlayer is in contact with the third layer.

Another embodiment of the present invention is a light-emitting elementincluding at least an EL layer between an anode and a cathode; a firstlayer including a high hole-transporting substance and an acceptorsubstance, provided in contact with the cathode between the cathode andthe EL layer; a second layer including a high electron-transportingsubstance, provided in contact with the first layer; and a third layerincluding a donor substance and a high electron-transporting substance,provided in contact with the second layer and the EL layer.

In the above structure, the first layer includes the acceptor substanceat a mass ratio of from 0.1 to 4.0 with respect to the highhole-transporting substance.

In the above structure, the first layer has a stacked structure of alayer including a high hole-transporting substance and a layer includingan acceptor substance.

In the above structure, the third layer includes the donor substance ata mass ratio of from 0.001 to 0.1 with respect to the highelectron-transporting substance.

In the above structure, the donor substance is an alkali metal, analkaline earth metal, a rare earth metal, an alkali metal compound, analkaline earth metal compound, or a rare earth metal compound.

In the above structure, the high electron-transporting substanceincluded in the second layer has a LUMO level of −5.0 eV or higher.

In the above structure, the high electron-transporting substanceincluded in the second layer is a perylene derivative ornitrogen-containing condensed aromatic compound.

In the above structure, the EL layer includes a fifth layer including ahigh hole-transporting substance and an acceptor substance, and thefifth layer is in contact with the anode.

In the above structure, the fifth layer includes the acceptor substanceat a mass ratio of from 0.1 to 4.0 with respect to the highhole-transporting substance.

In the above structure, the fifth layer has a stacked structure of alayer including a high hole-transporting substance and a layer includingan acceptor substance.

In the above structure, the acceptor substance included in the firstlayer is a transition metal oxide or an oxide of a metal belonging toGroups 4 to 8 in the periodic table.

In addition, in the above structure, the acceptor substance included inthe first layer is molybdenum oxide.

Further, the present invention includes, in its category, electronicdevices and lighting devices including light-emitting devices as well aslight-emitting devices including light-emitting elements. Accordingly,the term “light-emitting device” in this specification refers to imagedisplay devices, light-emitting devices, or light sources (includinglighting device). In addition, the light-emitting device includes alltypes modules: a module in which a light-emitting device is connected toa connector such as an FPC (Flexible Printed Circuit), a TAB (TapeAutomated Bonding) tape or a TCP (Tape Carrier Package), a module inwhich a printed wiring board is provided on the tip of a TAB tape or aTCP, and a module in which an IC (Integrated Circuit) is directlymounted on a light-emitting element using a COG (Chip On Glass)technique.

As described above, one embodiment of the present invention can providelight-emitting elements in which an increase of driving voltage can besuppressed. Further, light-emitting devices, electronic devices andlighting devices with reduced power consumption can be provided byincluding such light-emitting elements.

In addition, even when the thickness of the first layer is changed, thedriving voltage is not increased. Thus, even when the total thickness oflayers provided between electrodes of a light-emitting element ischanged, the light-emitting element with an increase of the drivingvoltage suppressed can be provided. Further, light-emitting devices withreduced power consumption can be provided by including suchlight-emitting elements.

In addition, even when the thickness of the first layer is madedifferent, the driving voltage is not increased. Thus, even when thethickness of layers provided between electrodes of a light-emittingelement is made different, the light-emitting element with suppressedrise of the driving voltage can be provided. Further, light-emittingdevices with reduced power consumption and excellent color purity can beprovided by including such light-emitting elements.

In addition, according to one embodiment of the present invention, thesecond layer is disposed between the first layer including an acceptorsubstance and a third layer including a high electron-injectingsubstance or a donor substance, and thus interaction between theacceptor substance and the high electron-injecting substance orinteraction between the acceptor substance and the donor substancehardly occurs, whereby a light-emitting element in which interactivefunctional interference hardly occurs can be provided. In addition, alight-emitting device with reduced power consumption can be provided byincluding such a light-emitting element.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B illustrate an element structure of a light-emittingelement and a band diagram thereof respectively;

FIGS. 2A and 2B illustrate an element structure of a light-emittingelement and a band diagram thereof respectively;

FIGS. 3A and 3B illustrate an element structure of a light-emittingelement and a band diagram thereof respectively;

FIGS. 4A and 4B illustrate element structures of light-emittingelements;

FIGS. 5A and 5B illustrate element structures of light-emittingelements;

FIGS. 6A to 6D illustrate a passive matrix light-emitting device;

FIG. 7 illustrates a passive matrix light-emitting device;

FIGS. 8A and 8B illustrate an active matrix light-emitting device;

FIGS. 9A to 9E illustrate electronic devices;

FIG. 10 illustrates a lighting device;

FIG. 11 shows characteristics of light-emitting elements of Examples;

FIG. 12 shows characteristics of the light-emitting elements ofExamples;

FIG. 13 shows characteristics of light-emitting elements of Examples;

FIG. 14 shows characteristics of the light-emitting elements ofExamples;

FIG. 15 shows characteristics of light-emitting elements of Examples;

FIG. 16 shows characteristics of the light-emitting elements ofExamples;

FIG. 17 shows characteristics of light-emitting elements of Examples;and

FIG. 18 shows characteristics of the light-emitting elements ofExamples.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to description to be given below, and it is tobe easily understood that modes and details thereof can be variouslymodified without departing from the spirit and the scope of the presentinvention.

Embodiment 1

Embodiment 1 will describe an element structure of a light-emittingelement according to one embodiment of the present invention withreference to FIGS. 1A and 1B.

In an element structure illustrated in FIG. 1A, an EL layer 103including an emission region is disposed between a pair of electrodes(an anode 101 and a cathode 102), a charge production region 106 as afirst layer which enables carrier production, an electron-relay layer105 as a second layer which receives and passes on electrons produced inthe charge production region 106, and an electron-injecting buffer 104as a third layer which injects the electrons received from theelectron-relay layer 105 into the EL layer 103 are sequentially stackedfrom the cathode 102 side between the cathode 102 and the EL layer 103.

In addition, in the charge production region 106, holes and electronsare produced as carriers of the light-emitting element, and the holesmove into the cathode 102 and the electrons move into the electron-relaylayer 105. Further, the electron-relay layer 105 has a highelectron-transporting property, and thus can transport rapidly electronsinto the electron-injecting buffer 104. Furthermore, because theelectron-injecting buffer 104 can reduce injection barrier in injectionof electrons into the EL layer 103, it can improve electron injectionefficiency into the EL layer 103.

FIG. 1B is a band diagram of the element structure illustrated in FIG.1A. In FIG. 1B, reference numeral 111 denotes a Fermi level of the anode101; 112, a Fermi level of the cathode 102; 113, a LUMO (lowestunoccupied molecular orbital) level of the EL layer 103; 114, a LUMOlevel of the electron-relay layer 105; and 115, an acceptor level of anacceptor substance in the charge production region 106.

In addition, the electron-relay layer 105 functions as a layer forefficiently injecting electrons produced in the charge production region106 into the EL layer 103, and thus the LUMO level of the electron-relaylayer 105 is formed so as to have a level between the acceptor level ofthe acceptor substance in the charge production region 106 and the LUMOlevel of the EL layer 103. Specifically, the LUMO level of theelectron-relay layer 105 is preferably about from −5.0 eV to −3.0 eV. Inaddition, the provision of the electron-relay layer 105 can preventinteraction between the charge production region 106 and theelectron-injecting buffer 104.

Further, as illustrated in the band diagram of FIG. 1B, the electronsthat have moved to the electron-relay layer 105 from the chargeproduction region 106 are easily injected into the LUMO level 113 of theEL layer 103 because of reduced injection barrier due to theelectron-injecting buffer 104. Note that the holes produced in thecharge production region 106 move into the cathode 102.

Next, materials that can be used for the above-described light-emittingelement are specifically described.

The anode 101 is preferably formed using a metal, an alloy, anelectrically-conductive compound, a mixture of these materials, or thelike, having a high work function (specifically, a work function ofgreater than or equal to 4.0 eV). Specifically, indium tin oxide (ITO),indium tin oxide containing silicon or silicon oxide, indium zinc oxide(IZO), indium oxide containing tungsten oxide and zinc oxide, and thelike can be given, for example.

Films of these conductive metal oxides are usually formed by sputtering.Alternatively, the films may be formed by application of a sol-gelmethod or the like. For example, a film of indium oxide-zinc oxide (IZO)can be formed by a sputtering method using a target in which zinc oxideis added to indium oxide at 1 wt % to 20 wt %. A film of indium oxidecontaining tungsten oxide and zinc oxide can be formed by a sputteringmethod using a target in which tungsten oxide and zinc oxide are addedto indium oxide at 0.5 wt % to 5 wt % and 0.1 wt % to 1 wt %,respectively.

Besides, as a material used for the anode 101, the following can begiven: gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium(Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium(Pd), titanium (Ti), nitride of a metal material (e.g., titaniumnitride), molybdenum oxide, vanadium oxide, ruthenium oxide, tungstenoxide, manganese oxide, titanium oxide, and the like. Alternatively, aconductive polymer such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS)or polyaniline/poly(styrenesulfonic acid) (PAni/PSS) may be used. Notethat, in the case where a charge production region is provided incontact with the anode 101, a variety of conductive materials such as Aland Ag can be used for the anode 101 regardless of their work functions.

The cathode 102 can be formed using a metal, an alloy, anelectrically-conductive compound, a mixture of these materials, or thelike, having a low work function (specifically, a work function of lessthan or equal to 3.8 eV). As specific examples of such a cathodematerial, the following can be given: an element that belongs to Group 1or 2 of the periodic table, that is, an alkali metal such as lithium(Li) or cesium (Cs), an alkaline earth metal such as magnesium (Mg),calcium (Ca), or strontium (Sr), an alloy containing these (such as anMgAg alloy or an AlLi alloy), a rare-earth metal such as europium (Eu)or ytterbium (Yb), an alloy containing these, and the like. Note that afilm of an alkali metal, an alkaline earth metal, or an alloy thereofcan be formed by a vacuum evaporation method. Alternatively, an alloycontaining an alkali metal or an alkaline earth metal can be formed by asputtering method. Further alternatively, a film can be formed usingsilver paste or the like by an ink-jet method or the like.

Alternatively, the cathode 102 can be formed using a stack of a thinfilm of an alkali metal compound, an alkaline earth metal compound, or arare earth metal compound (e.g., lithium fluoride (LiF), lithium oxide(LiOx), cesium fluoride (CsF), calcium fluoride (CaF₂), or erbiumfluoride (ErF₃)) and a film of a metal such as aluminum. However, in thecase where the charge production region is provided in contact with thecathode 102 as in the structure of this embodiment, a variety ofconductive materials such as Al, Ag, ITO, and indium oxide-tin oxidecontaining silicon or silicon oxide can be used for the cathode 102regardless of their work functions.

Note that in the light-emitting element described in this embodiment, atleast one of the anode and the cathode may have a property oftransmitting visible light. The light-transmitting property can beensured with use of a transparent electrode such as ITO, or reduction inthe thickness of an electrode.

The EL layer 103 may include at least a light-emitting layer, and mayalso have a structure in which layers other than the light-emittinglayer are stacked. As the layers other than the light-emitting layer,there are layers formed of a substance having a high hole-injectingproperty, a substance having a high hole-transporting property, asubstance having a high electron-transporting property, a substancehaving a high electron-injecting property, a substance having a bipolarproperty (a substance having high electron-and-hole-transportingproperties), and the like. Specifically, a hole-injecting layer, ahole-transporting layer, a light-emitting layer, a hole-blocking layer,an electron-transporting layer, an electron-injecting layer, and thelike are given, and such layers can be combined as appropriate andstacked from the anode side. Furthermore, a charge production region canbe provided in a portion of the EL layer 103, which is on the side wherethe EL layer 103 is in contact with the anode 101.

A material which is used for forming each of the layers included in theEL layer 103 is specifically described.

The hole-injecting layer is a layer containing a substance having a highhole-injecting property. As the substance having a high hole-injectingproperty, for example, molybdenum oxide, vanadium oxide, rutheniumoxide, tungsten oxide, manganese oxide, or the like can be used.Besides, a phthalocyanine-based compound such as phthalocyanine (H₂Pc)or copper phthalocyanine (CuPc), a high molecule such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS),or the like can also be used for forming the hole-injecting layer.

The hole-transporting layer is a layer containing a substance having ahigh hole-transporting property. As the substance having a highhole-transporting property, the following can be given, for example:aromatic amine compounds such as4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA), and4,4′-bis[N-(spiro-9,9′-bifluorene-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB);3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1);3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(1-naphtyl)-N-(9-phenylcarbazol-3-yl-amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and the like. Alternatively, the followingcarbazole derivative can be used: 4,4′-di(N-carbazolyl)biphenyl(abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene(abbreviation: TCPB), and 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA). The substances listed here are mainly substanceshaving a hole mobility of greater than or equal to 10⁻⁶ cm²/Vs. However,substances other than those can also be used as long as they have ahole-transporting property higher than an electron-transportingproperty. The layer containing a substance having a highhole-transporting property is not limited to a single layer, and may bea stack of two or more layers each containing the above-describedsubstance.

In addition to the above substances, a high molecular compound such aspoly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine)(abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide] (abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD) can be used for the hole-transporting layer.

The light-emitting layer is a layer including a light-emittingsubstance. As the light-emitting substance, the following fluorescentcompound can be used, for example:N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine(abbreviation: 2YGAPPA),N,9-diphenyl-N-[4-(10-phenyl-9-antryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA), perylene, 2,5,8,11-tetra-tert-butylperylene(abbreviation: TBP),4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA),N,N″-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′,N″-triphenyl-1,4-phenylenediamine] (abbreviation: DPABPA),N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: 2PCAPPA),N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPPA),N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine(abbreviation: DBC1), coumarin 30,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), coumarin 545T, N,N′-diphenylquinacridone(abbreviation: DPQd), rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile (abbreviation: DCM1),2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCM2),N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,14-diphenyl-N,N,N′,N″-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine (abbreviation: p-mPhAFD),2-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTI),2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTB),2-(2,6-bis{2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile(abbreviation: BisDCM),2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: BisDCJTM), and the like.

Alternatively, as the light-emitting substance, the followingphosphorescent compound can be used, for example:bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)picolinate(abbreviation: FIrpic),bis[2-(3′,5′-bistrifluoromethylphenyl)pyridinato-N,C^(2′)]iridium(III)picolinate(abbreviation: Ir(CF₃ppy)₂(pic)),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate(abbreviation: FIracac), tris(2-phenylpyridinato-N,C^(2′))iridium(III)(abbreviation: Ir(ppy)₃), bis(2-phenylpyridinato)iridium(III)acetylacetonato (abbreviation: Ir(ppy)₂(acac)),bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation:Ir(bzq)₂(acac)),bis(2,4-diphenyl-1,3-oxazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(dpo)₂(acac)),bis[2-(4′-perfluorophenylphenyl)pyridinato]iridium(III)acetylacetonate(abbreviation: Ir(p-PF-ph)₂(acac)),bis(2-phenylbenzothiazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(bt)₂(acac)),bis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C^(3′)]iridium(III)acetylacetonate (abbreviation: Ir(btp)₂(acac)),bis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(piq)₂(acac)),(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)),(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation:Ir(tppr)₂(acac)),2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrinplatinum(II) (abbreviation: PtOEP),tris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation:Tb(acac)₃(Phen)),tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)(abbreviation: Eu(DBM)₃(Phen)),tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: Eu(TTA)₃(Phen)), and the like.

Note that those light-emitting substances are preferably dispersed in ahost material. As the host material, for example, the following can beused: an aromatic amine compound such as NPB (abbreviation), TPD(abbreviation), TCTA (abbreviation), TDATA (abbreviation), MTDATA(abbreviation), or BSPB (abbreviation); a carbazole derivative such asPCzPCA1 (abbreviation), PCzPCA2 (abbreviation), PCzPCN1 (abbreviation),CBP (abbreviation), TCPB (abbreviation), or CzPA (abbreviation); a highhole-transporting substance which contains a high molecular compound,such as PVK (abbreviation), PVTPA (abbreviation), PTPDMA (abbreviation),or Poly-TPD (abbreviation); a metal complex having a quinoline skeletonor a benzoquinoline skeleton, such as tris(8-quinolinolato)aluminum(abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum(abbreviation: Almq₃), bis(10-hydroxybenzo[h]-quinolinato)beryllium(abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)(4-phenylphenolate)aluminum (abbreviation:BAlq); a metal complex having an oxazole-based or thiazole-based ligand,such as bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation:Zn(BOX)₂) or bis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (abbreviation:Zn(BTZ)₂); or a substance having a high electron-transporting property,such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole(abbreviation: PBD),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]carbazole (abbreviation:CO11), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen), orbathocuproine (abbreviation: BCP).

The electron-transporting layer is a layer including a highelectron-transporting substance. As the high hole-transportingsubstance, for example, a metal complex having a quinoline skeleton or abenzoquinoline skeleton, such as Alq (abbreviation), Almq₃(abbreviation), BeBq₂ (abbreviation), or BAlq (abbreviation) can beused. In addition to the above, a metal complex having an oxazole-basedor thiazole-based ligand, such as Zn(BOX)₂ (abbreviation) or Zn(BTZ)₂(abbreviation) can also be used. Furthermore, in addition to the abovemetal complexes, PBD (abbreviation), OXD-7 (abbreviation),CO11(abbreviation), TAZ (abbreviation), BPhen (abbreviation), BCP(abbreviation), or the like can also be used. The substances listed hereare mainly substances having an electron mobility of 10⁻⁶ cm²/Vs orhigher. Note that substances other than those may be used as long asthey have an electron-transporting property higher than ahole-transporting property. Furthermore, the electron-transporting layermay have a structure in which two or more layers formed of the abovesubstances are stacked, without limitation to a single-layer structure.

In addition to the above substances, a high molecular compound such asPF-Py (abbreviation) or PF-BPy (abbreviation) can be used for theelectron-transporting layer.

The electron-injecting layer is a layer including a highelectron-injecting substance. As the high electron-injecting substance,the following can be given: an alkali metal or an alkaline earth metalsuch as lithium (Li), cesium (Cs), calcium (Ca), lithium fluoride (LiF),cesium fluoride (CsF), and calcium fluoride (CaF₂), and a compoundthereof. Alternatively, a layer containing an electron-transportingsubstance and an alkali metal, an alkaline earth metal, or a compoundthereof (e.g., Alq containing magnesium (Mg)) can be used. Such astructure makes it possible to increase the efficiency of injection ofelectrons from the cathode 102.

As described above, a charge production region which is provided in aportion of the EL layer 103 closer to the side which is in contact withthe anode 101 is a region that contains a high hole-transportingsubstance and an acceptor substance. The charge production region maynot only include a high hole-transporting substance and an acceptorsubstance in the same film but also includes a stacked layer of a layercontaining a high hole-transporting substance and a layer containing anacceptor substance. However, in the case of the stacked-layer structureprovided on the anode side, the layer containing an acceptor substanceis in contact with the anode 101, while in the case of the stacked-layerstructure provided on the cathode side, the layer containing a highhole-transporting substance is in contact with the cathode 102.

The charge production region is formed, whereby the anode 101 can beformed without consideration of a work function of a material forforming the anode 101. In other words, not only a material having a highwork function but also a material having a low work function can be usedas the material for forming the anode 101.

As the acceptor substance that is used for the charge production region,a transition metal oxide and an oxide of a metal belonging to Groups 4to 8 of the periodic table can be given. Specifically, molybdenum oxideis particularly preferable. Note that molybdenum oxide has a lowhygroscopic property.

As the high hole-transporting substance used for the charge productionregion, any of a variety of organic compounds such as an aromatic aminecompound, a carbazole derivative, an aromatic hydrocarbon, and a highmolecular compound (such as an oligomer, a dendrimer, or a polymer) canbe used. Specifically, a substance having a hole mobility of 10⁻⁶ cm²/Vsor higher is preferable. However, substances other than those can alsobe used as long as they have a hole-transporting property higher than anelectron-transporting property.

Note that such layers are stacked in appropriate combination, wherebythe EL layer 103 can be formed. Further, as a formation method of the ELlayer 103, any of a variety of methods (e.g., a dry process and a wetprocess) can be selected as appropriate depending on a material to beused. For example, a vacuum evaporation method, an ink-jet method, aspin coating method, or the like can be used. Note that a differentformation method may be employed for each layer.

Further, between the cathode 102 and the EL layer 103, theelectron-injecting buffer 104, the electron-relay layer 105, and thecharge production region 106 are provided. The charge production region106 is formed in contact with the cathode 102, the electron-relay layer105 is formed in contact with the charge production region 106, and theelectron-injecting buffer 104 is formed in contact with and between theelectron-relay layer 105 and the EL layer 103.

The charge production region 106 is a region that contains a highhole-transporting substance and an acceptor substance. Note that thecharge production region 106 can be formed using a material similar tothe above-described material used for the charge production region thatcan be formed in part of the EL layer 103 and can have a similarstructure to the charge production region. Therefore, the chargeproduction region 106 can not only contain a high hole-transportingsubstance and an acceptor substance in the same film but also include astacked layer of a layer containing a high hole-transporting substanceand a layer containing an acceptor substance. Note that in the case ofthe stacked layer, the layer containing a high hole-transportingsubstance is in contact with the cathode 102.

Note that the acceptor substance is preferably added to the chargeproduction region 106 so that the mass ratio of the acceptor substanceto the high hole-transporting substance is from 0.1 to 4.0:1.

The electron-relay layer 105 is a layer that can quickly receiveelectrons drawn out by the acceptor substance in the charge productionregion 106. Therefore, the electron-relay layer 105 is a layer thatcontains a high electron-transporting substance and is formed to have aLUMO level between the acceptor level of the acceptor in the chargeproduction region 106 and the LUMO level of the EL layer 103.Specifically, the LUMO level is from −5.0 eV to −3.0 eV is preferable.As the substance used for the electron-relay layer 105, for example, aperylene derivative and a nitrogen-containing condensed aromaticcompound can be given. Note that a nitrogen-containing condensedaromatic compound is preferably used for the electron-relay layer 105because of its stability. Furthermore, of nitrogen-containing condensedaromatic compounds, a compound having an electron-withdrawing group suchas a cyano group or a fluoro group is preferably used, in which caseelectrons are easily received in the electron-relay layer 105.

As specific examples of the perylene derivative, the following can begiven: 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA),3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI),N,N′-dioctyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C8H),N,N′-dihexyl-3,4,9,10-perylenetetracarboxylic diimide (Hex PTC), and thelike.

As specific examples of the nitrogen-containing condensed aromaticcompound, the following can be given:pirazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile (PPDN),2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (HAT(CN)₆),2,3-diphenylpyrido[2,3-b]pyrazine (2PYPR),2,3-bis(4-fluorophenyl)pyrido[2,3-b]pyrazine (F2PYPR), and the like.

Besides, 7,7,8,8-tetracyanoquinodimethane (TCNQ),1,4,5,8-naphthalenetetracarboxylicdianhydride (NTCDA),perfluoropentacene, copper hexadecafluoro phthalocyanine (F₁₆CuPc),N,N′-bis(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl-1,4,5,8-naphthalenetetracarboxylicdiimide (NTCDI-C8F),3′,4′-dibutyl-5,5″-bis(dicyanomethylene)-5,5″-dihydro-2,2′:5′,2″-terthiophen)(DCMT), methanofullerene, such as [6,6]-phenyl C₆₁ butyric acid methylester (PCBM), or the like can be used for the electron-relay layer 105.

The electron-injecting buffer 104 is a layer that can inject theelectrons received by the electron-relay layer 105 into the EL layer103. The provision of the electron-injecting buffer 104 makes itpossible to reduce the injection barrier between the charge productionregion 106 and the EL layer 103; thus, the electrons produced in thecharge production region 106 can be easily injected into the EL layer103.

A high electron-injecting substance can be used for theelectron-injecting buffer 104: for example, an alkali metal, an alkalineearth metal, a rare earth metal, a compound of the above metal(including an alkali metal compound (an oxide such as lithium oxide orthe like, a halide, and carbonate such as lithium carbonate or cesiumcarbonate), an alkaline earth metal compound (including an oxide, ahalide, and carbonate), and a rare earth metal compound (including anoxide, a halide, and carbonate).

Further, in the case where the electron-injecting buffer 104 contains ahigh electron-transporting substance and a donor substance, the donorsubstance is preferably added so that the mass ratio of the donorsubstance to the electron-transporting substance is from 0.001 to 0.1:1.Note that as the donor substance, an organic compound such astetrathianaphthacene (abbreviation: TTN), nickelocene, ordecamethylnickelocene can be used as well as an alkali metal, analkaline earth metal, a rare earth metal, a compound of the above metal(e.g., an alkali metal compound (including an oxide of lithium oxide orthe like, a halide, and carbonate such as lithium carbonate or cesiumcarbonate), an alkaline earth metal compound (including an oxide, ahalide, and carbonate), and a rare earth metal compound (including anoxide, a halide, and carbonate). Note that as the highelectron-transporting substance, a material similar to theabove-described material for the electron-transporting layer that can beformed in part of the EL layer 103 can be used.

The light-emitting element described in this embodiment can befabricated by combination of the above-described materials. Althoughlight emission from the above-described light-emitting substance can beobtained with this light-emitting element, a variety of emission colorscan be obtained by changing the type of the light-emitting substance. Inaddition, a plurality of light-emitting substances of different colorsare used as the light-emitting substance, whereby light emission havinga broad spectrum or white light emission can also be obtained. Note thatin the case of white light emission, plural layers which emit lightwhose colors are complementary colors can be stacked. Specific examplesof complementary colors include “blue and yellow”, “blue-green and red”,and the like.

Further, the light-emitting element described in this embodiment can beformed over any of a variety of substrates. As the substrate, forexample, a substrate made of glass, plastic, a metal plate, metal foil,or the like can be used. In the case where light emission of thelight-emitting element is extracted from the substrate side, a substratehaving a light-transmitting property may be used. Note that as thesubstrate, a substrate other than the above may be used as long as itcan serve as a support in the fabrication process of the light-emittingelement.

Note that a passive matrix light-emitting device in which bothelectrodes are formed in a grid pattern over the same substrate can bemanufactured with the structure of the light-emitting element describedin this embodiment. In addition, an active matrix light-emitting deviceincluding a light-emitting element which is electrically connected to athin film transistor (TFT) functioning as a switch, or the like and thedriving of which is controlled by the TFT can also be manufactured withthe structure of the light-emitting element described in thisembodiment. Note that the structure of the TFT is not particularlylimited. A staggered TFT or an inverted staggered TFT may be employed.In addition, a driver circuit formed with a TFT may be formed using ann-type TFT and a p-type TFT, or using either an n-type TFT or a p-typeTFT. Crystallinity of a semiconductor film used for the TFT is notparticularly limited, either. An amorphous semiconductor film may beused, or a crystalline semiconductor film may be used. Alternatively, asingle crystal semiconductor film may be used. The single crystalsemiconductor film can be formed by a Smart Cut (registered trademark)method or the like. Further alternatively, an oxide semiconductor, forexample, an oxide semiconductor containing indium, gallium, and zinc canbe used.

Further, the light-emitting element described in this embodiment can beformed by any of a variety of methods regardless of whether it is a dryprocess (e.g., a vacuum evaporation method) or a wet process (e.g., anink-jet method or a spin coating method).

The element structure described in this embodiment is employed, wherebythe driving voltage of the light-emitting element can be less likely tobe adversely affected by the thickness of the charge production region106. Thus, an increase in the driving voltage of the light-emittingelement can be suppressed, and improvement of color purity by opticaladjustment can be realized.

In addition, when the element structure described in this embodiment isemployed, the electron-relay layer 105 is disposed between the chargeproduction region 106 and the electron-injecting buffer 104, in whichcase a structure can be obtained in which the acceptor substancecontained in the charge production region 106 and the donor substancecontained in the electron-injecting buffer 104 are less likely tointeract with each other, and thus hardly interfere with theirfunctions.

Embodiment 2

In Embodiment 2, an example of the light-emitting element included inthe basic structure described in Embodiment 1 will be described withreference to FIGS. 2A and 2B.

As illustrated in FIG. 2A, in a light-emitting element described in thisembodiment, the EL layer 103 including a light-emitting region isdisposed between a pair of electrodes (the anode 101 and the cathode102), and between the EL layer 103 and the cathode 102, the chargeproduction region 106, the electron-relay layer 105 and theelectron-injecting buffer 104 are stacked in this order from the cathode102 side.

The anode 101, the cathode 102, the EL layer 103, the charge productionregion 106, and the electron-relay layer 105 in Embodiment 2 can beformed using materials similar to those described in Embodiment 1.

In addition, as a substance used for the electron-injecting buffer 104,the following can be given: substances having a high electron-injectingproperty, such as alkali metals such as lithium (Li) and cesium (Cs);alkaline earth metals such as magnesium (Mg), calcium (Ca), andstrontium (Sr); rare earth metals such as europium (Eu) and ytterbium(Yb); alkali metal compounds (including an oxide of lithium oxide andthe like, a halide, and carbonate such as lithium carbonate and cesiumcarbonate); alkaline earth metal compounds (including an oxide, ahalide, and carbonate), and rare earth metal compounds (including anoxide, a halide, and carbonate); and the like.

In the light-emitting element described in this embodiment, the EL layer103 is formed over the anode 101, then, the electron-injecting buffer104, the electron-relay layer 105, and the charge production region 106are sequentially formed. The electron-injecting buffer 104 is formed toa very small thickness (specifically, 1 nm or smaller) so that anincrease in the driving voltage is prevented. Thus, theelectron-injecting buffer 104 is proximately located at the interfacebetween the electron-relay layer 105 and the electron-transporting layer107, which is a part of the EL layer 103. However, in a case where theelectron-injecting buffer 104 is formed over the electron-transportinglayer 107 after the electron-transporting layer 107 is formed, part ofthe substance used for forming the electron-injecting buffer 104 canalso exist in the electron-transporting layer 107 that is a part of theEL layer 103.

FIG. 2B is a band diagram of the element structure of FIG. 2A from theanode 101 side. In other words, the electron-injecting buffer 104 isprovided at the interface between the electron-relay layer 105 and theEL layer 103, whereby the injection barrier between the chargeproduction region 106 and the EL layer 103 can be reduced; thus,electrons produced in the charge production region 106 can be easilyinjected into the EL layer 103. In addition, holes produced in thecharge production region 106 move into the cathode.

The structure of the electron-injecting buffer described in thisembodiment is employed, whereby the driving voltage of thelight-emitting element can be reduced in comparison with a structure ofan electron-injecting buffer described in Embodiment 3 (that is formedby addition of a donor substance to a high electron-transportingsubstance).

In this embodiment, among the above-described high electron-injectingsubstances used for the electron-injecting buffer 104, an alkali metalcompound (including an oxide such as lithium oxide, a halide, andcarbonate such as lithium carbonate or cesium carbonate), an alkalineearth metal compound (including an oxide, a halide, and carbonate), arare earth metal compound (including an oxide, a halide, and carbonate),are the like are stable in air. Thus, the light-emitting element of thisembodiment using such a substance is suitable for mass production.

Note that the structure described in Embodiment 2 can be combined withthe structure in Embodiment 1 as appropriate.

Embodiment 3

In Embodiment 3, an example of the light-emitting element included inthe basic structure described in Embodiment 1 will be described withreference to FIGS. 3A and 3B.

As illustrated in FIG. 3A, in a light-emitting element described in thisembodiment, the EL layer 103 including a light-emitting region isdisposed between a pair of electrodes (the anode 101 and the cathode102), and between the EL layer 103 and the cathode 102, the chargeproduction region 106, the electron-relay layer 105, and theelectron-injecting buffer 104 are stacked sequentially from the cathode102 side. In addition, the electron-injecting buffer 104 contains a highelectron-transporting substance and a donor substance.

Note that, in this embodiment, the donor substance is preferably addedso that the mass ratio of the donor substance to the highelectron-transporting substance is from 0.001 to 0.1:1. Accordingly, theelectron-injecting buffer 104 can function as electron-injecting buffer.

The anode 101, the cathode 102, the EL layer 103, the charge productionregion 106, and the electron-relay layer 105 in Embodiment 3 can beformed using materials similar to those described in Embodiment 1.

In addition, as the high electron-transporting substance used for theelectron-injecting buffer 104, the following can be used, for example: ametal complex having a quinoline skeleton or a benzoquinoline skeleton,such as tris(8-quinolinolato)aluminum (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAlq), or the like. Alternatively, a metal complex having anoxazole-based or thiazole-based ligand, such asbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (abbreviation: Zn(BTZ)₂)can be used. Further alternatively, besides the metal complexes,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]carbazole (abbreviation:CO11), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-biphenylyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or the like can be used. Thesubstances described here are mainly substances having an electronmobility of 10⁻⁶ cm²/Vs or higher.

Besides the above-described substances, a high molecular compound suchas poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py) andpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy) can be used.

Further, as the donor substance used for the electron-injecting buffer104, an alkali metal, an alkaline earth metal, a rare earth metal, acompound thereof (including an alkali metal compound (including an oxidesuch as lithium oxide, a halide, and carbonate such as lithium carbonateor cesium carbonate), an alkaline earth metal compound (including anoxide, a halide, and carbonate), a rare earth metal compound (includingan oxide, a halide, and carbonate)), or the like can be used.Furthermore, an organic compound such as tetrathianaphthacene(abbreviation: TTN), nickelocene, or decamethylnickelocene can be used.

Note that, in this embodiment, the high electron-transporting substanceused for the electron-injecting buffer 104 and a highelectron-transporting substance used for the electron-transporting layer107 that is a part of the EL layer 103 may be the same or different.

As illustrated in FIG. 3A, the light-emitting element described in thisembodiment has a feature in that the electron-injecting buffer 104containing the high electron-transporting substance and the donorsubstance is formed between the EL layer 103 and the electron-relaylayer 105. FIG. 3B is a band diagram of this element structure.

In other words, the electron-injecting buffer 104 is formed, whereby theinjection barrier between the electron-relay layer 105 and the EL layer103 can be reduced; thus, electrons produced in the charge productionregion 106 can be easily injected into the EL layer 103. In addition,holes produced in the charge production region 106 move to the cathode.

Note that the structure described in Embodiment 3 can be combined withthe structure described in Embodiment 1 or 2 as appropriate.

Embodiment 4

In Embodiment 4, as an example of the light-emitting element included inthe basic structure described in Embodiment 1, the structure of thecharge production region 106 will be described with reference to FIGS.4A and 4B.

In element structures illustrated in FIGS. 4A and 4B, the EL layer 103including a light-emitting region is disposed between a pair ofelectrodes (the anode 101 and the cathode 102), and between the EL layer103 and the cathode 102, the charge production region 106, theelectron-relay layer 105, and the electron-injecting buffer 104 arestacked sequentially from the cathode 102 side. In addition, the anode101, the cathode 102, the EL layer 103, the electron-relay layer 105,and the electron-injecting buffer 104 can be formed using materialssimilar to those described in Embodiment 1.

In the element structures illustrated in FIGS. 4A and 4B, the chargeproduction region 106 is a region that contains a high hole-transportingsubstance and an acceptor substance. Note that in the charge productionregion 106, electrons are extracted from the high hole-transportingsubstance by the acceptor substance, whereby holes and electrons areproduced.

The charge production region 106 illustrated in FIG. 4A has a structurein which a high hole-transporting substance and an acceptor substanceare contained in the same film. In that case, the acceptor substance ispreferably added so that the mass ratio of the acceptor substance to thehigh hole-transporting substance is from 0.1 to 4.0:1, in which casecarriers are easily produced in the charge production region 106.

On the other hand, the charge production region 106 illustrated in FIG.4B has a structure in which a layer including a high hole-transportingsubstance 106 a and a layer including an acceptor substance 106 b arestacked. A charge transfer complex produced in the charge productionregion 106 is a substance which absorbs light in visible region.However, in the stacked structure in which the layer including a highhole-transporting substance 106 a and the layer including an acceptorsubstance 106 b are stacked, the charge transfer complex is formed atthe interface between the layer 106 a and the layer 106 b, not in thewhole charge production region 106. In other words, when the chargeproduction region 106 has such a stacked structure, light emission inthe light-emitting element is hardly subjected to influence of thecharge transfer complex, which is preferable.

As the high hole-transporting substance used for the charge productionregion 106, any of a variety of organic compounds such as an aromaticamine compound, a carbazole derivative, aromatic hydrocarbon, and a highmolecular compound (such as an oligomer, a dendrimer, and a polymer) canbe used. Specifically, a substance having a hole mobility of 10⁻⁶ cm²/Vsor higher is preferable. However, substances other than those can alsobe used as long as they have a hole-transporting property higher than anelectron-transporting property.

As specific examples of the aromatic amine compound, the following canbe given: 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation:NPB or α-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA) 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),N,N′-bis(4-methylphenyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation:DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB), 4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B), and the like.

As specific examples of the carbazole derivative, the following can begiven: 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl-amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and the like. Besides, the following can begiven: 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA),1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and thelike.

As specific examples of the aromatic hydrocarbon, the following can begiven: 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation:t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA);9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth); 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),9,10-bis[2-(1-naphthyl)phenyl]-2-tert-butylanthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl;10,10′-diphenyl-9,9′-bianthryl;10,10′-bis(2-phenylphenyl)-9,9′-bianthryl;10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl; anthracene;tetracene; rubrene; perylene; 2,5,8,11-tetra(tert-butyl)perylene; andthe like. In addition to those, pentacene, coronene, or the like canalso be used. In this way, the aromatic hydrocarbon having a holemobility of 1×10⁻⁶ cm²/Vs or higher and 14 to 42 carbon atoms is morepreferably used.

Further, the aromatic hydrocarbon may have a vinyl skeleton. As thearomatic hydrocarbon having a vinyl group, for example,4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi),9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA),and the like can be given.

Moreover, a high molecular compound such as poly(N-vinylcarbazole)(abbreviation: PVK) or poly(4-vinyltriphenylamine) (abbreviation: PVTPA)can also be used.

As the acceptor substance used for the charge production region 106,7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and the like can be given. In addition, atransition metal oxide can be given. Moreover, an oxide of a metalbelonging to any of Groups 4 to 8 of the periodic table can be given.Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromiumoxide, molybdenum oxide, tungsten oxide, manganese oxide, and rheniumoxide are preferable because of their high electron acceptingproperties.

Note that the structure described in Embodiment 4 can be combined withany of the structures described in Embodiments 1 to 3 as appropriate.

Embodiment 5

Embodiment 5 will describe a structure in which a first chargeproduction region is formed in a part of the EL layer 103, so as to bein contact with the anode 101 with reference to FIGS. 5A and 5B, as anexample of the light-emitting element included in the basic structuredescribed in Embodiment 1. Thus, in this embodiment, the chargeproduction region which is a part of the EL layer 103 and formed incontact with the anode 101 in this embodiment is referred to as a firstcharge production region 108, and the charge production region 106described in Embodiment 1 is referred to as a second charge productionregion 116.

In FIGS. 5A and 5B, the EL layer 103 including a light-emitting regionis disposed between a pair of electrodes (the anode 101 and a cathode102), and the first charge production region 108 which is formed in apart of the EL layer 103 so as to be in contact with the anode 101 isprovided. In addition, the second charge production region 116, theelectron-relay layer 105 and the electron-injecting buffer 104 aresequentially stacked from the cathode 102 side between the cathode 102and the EL layer 103. Note that the materials described in Embodiments 1to 4 can be used for the anode 101, the cathode 102, the EL layer 103,the electron-injecting buffer 104, and the electron-relay layer 105, andthe materials which are described in Embodiments 1 to 4 as the materialsfor the charge production region 106 can be used for the second chargeproduction region 116.

In the light-emitting element illustrated in FIGS. 5A and 5B, the firstcharge production region 108 is similar to the second charge productionregion 116, and includes a high hole-transporting substance and anacceptor substance. Thus, in the first charge production region 108,electrons are extracted from the high hole-transporting substance by theacceptor substance, whereby holes and electrons are produced.

The first charge production region 108 illustrated in FIG. 5A has astructure in which a high hole-transporting substance and an acceptorsubstance are included in one film. In this case, preferably, theacceptor substance is added to the first charge production region 108 sothat a mass ratio of the acceptor substance to the highhole-transporting substance is from 0.1 to 4.0:1, whereby production ofcarriers in the first charge production region 108 is facilitated. Inaddition, in FIG. 5A, the first charge production region 108 and thesecond charge production region 116 are formed using the same material,and thereby stresses on the anode 101 side and the cathode 102 side ofthe light-emitting element can be even, which leads to release ofexternal stress.

On the other hand, the first charge production region 108 illustrated inFIG. 5B has a structure in which a layer including a highhole-transporting substance 108 a and a layer including an acceptorsubstance 108 b are stacked. Note that a charge transfer complexproduced in the first charge production region 108 is a substance whichabsorbs light in visible region, but in a case where the layer includinga high hole-transporting substance 108 a and the layer including theacceptor substance 108 b are stacked, the charge transfer complex existsnot in the whole of the first charge production region 108 but at theinterface between the layer including the high hole-transportingsubstance 108 a and the layer including the acceptor substance 108 b. Inother words, when the first charge production region 108 has a stackedstructure, light emission in the light-emitting element is hardlyinfluenced by the charge transfer complex, which is preferable. Inaddition, as illustrated in FIG. 5B, the structure of the second chargeproduction region 116 may be a structure in which a layer including ahigh hole-transporting substance 116 a and a layer including an acceptorsubstance 116 b are stacked.

Note that as the high hole-transporting substance used for formation ofthe first charge production region 108, the substances described as thehigh hole-transporting substance used for the formation of the chargeproduction region 106 in Embodiment 4 can be used similarly. Inaddition, as the acceptor substance used for formation of the firstcharge production region 108, the substances described as the acceptorsubstance used for formation of the charge production region 106 inEmbodiment 4 can be used similarly.

Note that the structure shown in Embodiment 5 can be combined with anyof the structures described in Embodiments 1 to 4 as appropriate.

Embodiment 6

Embodiment 6 will describe a passive matrix light-emitting device and anactive matrix light-emitting device as examples of light-emittingdevices manufactured using such a light-emitting element described inany of Embodiments 1 to 5.

FIGS. 6A to 6D and FIG. 7 illustrate examples of passive matrixlight-emitting devices.

In a passive-matrix (also referred to as “simple-matrix”) light-emittingdevice, a plurality of anodes aligned in stripes (in stripe form) areprovided to be perpendicular to a plurality of cathodes aligned instripes, and a light-emitting layer is disposed at each intersectionportion between the anode and the cathode. Therefore, a pixel at anintersection of an anode selected (to which a voltage is applied) and acathode selected emits light.

FIGS. 6A to 6C are top views of a pixel portion before sealing. FIG. 6Dis a cross-sectional view taken along a chain line A-A′ in FIGS. 6A to6C.

Over a substrate 601, an insulating layer 602 is formed as a baseinsulating layer. Note that the insulating layer is not necessarilyformed if not needed. Over the insulating layer 602, a plurality offirst electrodes 603 are disposed in stripe form with equal spacingtherebetween (FIG. 6A).

A partition 604 having openings corresponding to respective pixels isprovided over the first electrodes 603. The partition 604 havingopenings is formed of an insulating material (a photosensitive ornon-photosensitive organic material (e.g., polyimide, acrylic,polyamide, polyimideamide, resist, or benzocyclobutene) or an SOG film(e.g., a SiO_(x) film containing an alkyl group)). Note that an opening605 corresponding to each pixel serves as a light-emitting region (FIG.6B).

Over the partition 604 having openings, a plurality of parallelinversely tapered partitions 606 are provided to intersect with thefirst electrodes 603 (FIG. 6C). The inversely tapered partitions 606 areformed as follows. By a photolithography method, a positive-typephotosensitive resin of which portion unexposed to light remains as apattern is used, and the amount of light exposure or the length ofdevelopment time is adjusted so that a lower portion of the pattern isetched more.

After formation of the inversely tapered partitions 606 as illustratedin FIG. 6C, a layer including an organic compound 607 and a secondelectrode 608 are sequentially formed as illustrated in FIG. 6D. Notethat the layer including an organic compound 607 in this embodimentindicates the layer including an EL layer, a charge production region(including the first charge production region and the second chargeproduction region), an electron-relay layer, and an electron-injectingbuffer, described as the layer formed between the anode and the cathodein Embodiments 1 to 5. The total height of the partition 604 havingopenings and the inversely tapered partition 606 is designed to belarger than the total thickness of the layer including an organiccompound 607 and the second electrode 608, and thus, as illustrated inFIG. 6D, divided plural regions each including the layers including anorganic compound 607 and second electrodes 608 are formed. Note that theplurality of divided regions are electrically isolated from one another.

The second electrodes 608 are electrodes in stripe form which areparallel to each other and extend in the direction intersecting with thefirst electrodes 603. Note that the layer including an organic compound607 and part of a conductive layer forming the second electrode 608 arealso formed over the inversely tapered partitions 606; however, they areseparated from the layer including an organic compound 607 and thesecond electrodes 608. Note that the layer including an organic compoundin this embodiment includes the charge production region (including thefirst charge production region and the second charge production region),the electron-relay layer, the electron-injecting buffer, and the ELlayer described in Embodiments 1 to 5, and the EL layer includes atleast a light-emitting layer.

Note that there is no particular limitation on the first electrode 603and the second electrode 608, and it is acceptable that one of the firstelectrode 603 and the second electrode 608 in this embodiment may be ananode, and the other may be cathode. In addition, the stack structure ofthe layer including an organic compound 607 may be adjusted asappropriate depending on the polarities of electrodes so as to have thestructure as described in Embodiments 1 to 5.

In addition, if necessary, a sealing material such as a sealing can or aglass substrate may be attached to the substrate 601 by an adhesiveagent for sealing so that the light-emitting element can be disposed inthe sealed space. Thereby, deterioration of the light-emitting elementcan be prevented. The sealed space may be filled with filler or a dryinert gas. Further, a desiccant or the like may be put between thesubstrate and the sealing material to prevent deterioration of thelight-emitting element due to moisture. The desiccant removes a minuteamount of moisture, thereby achieving sufficient desiccation. Thedesiccant may be a substance which absorbs moisture by chemicaladsorption such as an oxide of an alkaline earth metal as typified bycalcium oxide or barium oxide. Additionally, a substance which adsorbsmoisture by physical adsorption such as zeolite or silica gel may beused as well, as a desiccant.

Next, FIG. 7 shows a top view when the passive-matrix light-emittingdevice in FIGS. 6A to 6D is mounted with an FPC or the like.

As illustrated in FIG. 7, in a pixel portion forming an image display, agroup of scan lines and a group of data lines intersect with each otherso as to be perpendicular.

The first electrode 603 in FIGS. 6A to 6D corresponds to a scan line 703in FIG. 7, and the second electrode 608 in FIGS. 6A to 6D corresponds toa data line 708 in FIG. 7, and the inversely tapered partition 606 inFIGS. 6A to 6D corresponds to a partition 706 in FIG. 7. The layersincluding an organic compound 607 in FIGS. 6A to 6D are disposed betweenthe data lines 708 and the scan lines 703, and an intersection portionindicated by a region 705 corresponds to one pixel.

It is to be noted that the scan lines 703 are electrically connected attheir ends to connection wirings 709 and the connection wirings 709 areconnected to an FPC 711 b through an input terminal 710. In addition,the data line 708 is connected to an FPC 711 a via an input terminal712.

If necessary, a polarizing plate, a circularly polarizing plate(including an elliptically polarizing plate), a retardation plate (aquarter-wave plate or a half-wave plate), or an optical film such as acolor filter may be appropriately provided for an emission surface oflight emitted from the light-emitting layer. Further, the polarizingplate or the circularly polarizing plate may be provided with ananti-reflection film. For example, anti-glare treatment may be done, bywhich reflected light can be diffused by projections and depressions onthe surface so as to reduce the glare.

Although FIG. 7 illustrates the example in which a driver circuit is notprovided over the substrate, an IC chip including a driver circuit maybe mounted on the substrate.

When the IC chip is mounted, a data line side IC and a scan line sideIC, in each of which a driver circuit for transmitting a signal to apixel portion is formed, are mounted on the periphery of the pixelportion (outside the pixel portion) by a COG method. The mounting may beperformed using TCP or a wire bonding method other than the COG method.TCP is a TAB tape mounted with an IC, and a TAB tape is connected to awiring over an element formation substrate so that an IC is mounted.Each of the data line side IC and the scan line side IC may be formedusing a silicon substrate. Alternatively, it may be formed in such amanner that a driver circuit is formed using a TFT over a glasssubstrate, a quartz substrate, or a plastic substrate.

Next, an example of an active-matrix light-emitting device is describedwith reference to FIGS. 8A and 8B. Note that FIG. 8A is a top viewillustrating a light-emitting device and FIG. 8B is a cross-sectionalview taken along the chain line A-A′ in FIG. 8A. The active-matrixlight-emitting device of this embodiment includes a pixel portion 802provided over an element substrate 801, a driver circuit portion (asource-side driver circuit) 803, and a driver circuit portion (agate-side driver circuit) 804. The pixel portion 802, the driver circuitportion 803 and the driver circuit portion 804 are sealed between theelement substrate 801 and the sealing substrate 806 by the sealingmaterial 805.

In addition, over the element substrate 801, a lead wiring 807 forconnecting an external input terminal which transmits a signal (e.g., avideo signal, a clock signal, a start signal, or a reset signal) or anelectric potential to the driver circuit portion 803 and the drivercircuit portion 804 is provided. Here, an example is described in whicha flexible printed circuit (FPC) 808 is provided as the external inputterminal. Although only an FPC is shown here, this FPC may have aprinted wiring board (PWB) attached. The “light-emitting device” in thisspecification includes not only a light-emitting device body but also alight-emitting device to which an FPC or a PWB is attached.

Next, the sectional structure will be described with reference to FIG.8B. Although the driver circuit portions and the pixel portion areformed over the element substrate 801, the pixel portion 802 and thedriver circuit portion 803 which is the source side driver circuit areillustrated here.

An example is illustrated in which a CMOS circuit which is a combinationof an n-channel TFT 809 and a p-channel TFT 810 is formed as the drivercircuit portion 803. Note that a circuit included in the driver circuitportion may be formed using various types of circuits such as a CMOScircuit, a PMOS circuit, or an NMOS circuit. Although a driverintegrated type in which the driver circuit is formed over the substrateis described in this embodiment, the driver circuit may not necessarilybe formed over the substrate, and the driver circuit can be formedoutside, not over the substrate.

Further, the pixel portion 802 has a plurality of pixels, each includinga switching TFT 811, a current-controlling TFT 812, and an anode 813electrically connected to a wiring (a source electrode or a drainelectrode) of the current-controlling TFT 812. An insulator 814 isformed so as to cover an end portion of the anode 813. In thisembodiment, the insulator 814 is formed using a positive photosensitiveacrylic resin.

The insulator 814 is preferably formed so as to have a curved surfacewith curvature at an upper end portion or a lower end portion thereof inorder to obtain favorable coverage by a film which is to be stacked overthe insulator 814. For example, in a case of using a positivephotosensitive acrylic resin as a material for the insulator 814, theinsulator 814 is preferably formed so as to have a curved surface with acurvature radius (0.2 to 3 μm) at the upper end portion thereof. Eithera negative photosensitive material which becomes insoluble in an etchantby light or a positive photosensitive material which becomes soluble inan etchant by light can be used for the insulator 814. As the insulator814, not only an organic compound but also an inorganic compound such assilicon oxide or silicon oxynitride can be used.

A layer including an organic compound 815 and a cathode 816 are stackedover the anode 813. Note that when an ITO film is used as the anode 813,and a stacked film of a titanium nitride film and a film containingaluminum as its main component or a stacked film of a titanium nitridefilm, a film containing aluminum as its main component, and a titaniumnitride film is used as a wiring of the current-controlling TFT 812which is connected to an anode 813, resistance of the wiring is low andfavorable ohmic contact with the ITO film can be obtained. Note that,although not illustrated here, the cathode 816 is electrically connectedto the FPC 808 which is an external input terminal.

In addition, the layer including an organic compound 815 described inthis embodiment is a layer including the EL layer, the charge productionregion (including a first charge production region and a second chargeproduction region), the electron-relay layer, and the electron-injectingbuffer, which is described as the layer formed between the anode and thecathode in Embodiments 1 to 5. In the EL layer, at least thelight-emitting layer is provided, and in addition to the light-emittinglayer, a hole-injecting layer, a hole-transporting layer, anelectron-transporting layer, and/or an electron-injecting layer are/isprovided as appropriate. The stacked structure of the anode 813, thelayer including an organic compound 815 and the cathode 816 correspondsto a light-emitting element 817.

Although only one light-emitting element 817 is illustrated in thecross-sectional view of FIG. 8B, a plurality of light-emitting elementsare arranged in matrix in the pixel portion 802. Therefore, in the caseof color display with color elements of R (red) G (green) and B (blue),plural light-emitting elements for emission colors of three colors (R,G, B) are formed in the pixel portion 802. In addition, the number ofcolor elements is not limited to three, and four or more colors may beused or another color than RGB may be used. For example, RGBW (W meanswhite) display becomes possible by addition of white.

As methods for fabricating light-emitting elements having differentcolor elements, there are a method in which an EL layer for each coloris separately formed, a method in which all EL layers for white emissionare formed and color filters are used in combination to obtainlight-emitting elements for different color elements, and a method inwhich all EL layers for blue emission or emission with a wavelengthshorter than blue are formed and color conversion layers are used incombination to obtain light-emitting elements for different colorelements, and the like.

Furthermore, the sealing substrate 806 and the element substrate 801 areattached to each other with the sealing material 805, whereby alight-emitting element 817 is provided in the space 818 surrounded bythe element substrate 801, the sealing substrate 806, and the sealingmaterial 805. Note that the space 818 may be filled with an inert gas(such as nitrogen or argon) or the sealing material 805.

It is preferable to use an epoxy resin for the sealing material 805. Inaddition, such a material as used for the sealing material 805 does nottransmit moisture and oxygen as much as possible. As the sealingsubstrate 806, a plastic substrate formed of FRP (Fiberglass-ReinforcedPlastics), PVF (polyvinyl fluoride), polyester, acrylic, or the like canbe used besides a glass substrate or a quartz substrate.

As described above, an active matrix light-emitting device can beobtained.

Note that the structure described in Embodiment 6 can be combined withany structure of Embodiments 1 to 5 as appropriate.

Embodiment 7

In Embodiment 7, a variety of electronic devices and lighting deviceswhich are manufactured using a light-emitting device formed according toone embodiment of the present invention are described using FIGS. 9A to9E.

Examples of such electronic devices for which a light-emitting deviceaccording to one embodiment is used include television sets (also calledTV or television receivers), monitors for computers or the like, camerassuch as digital cameras or digital video cameras, digital photo frames,mobile phones (also called cellular phones or portable telephones),portable game machines, portable information terminals, audio playbackdevices, and large game machines such as pachinko machines. Specificexamples of these electronic devices and lighting devices areillustrated in FIGS. 9A to 9E.

FIG. 9A illustrates an example of a television set 9100. In thetelevision set 9100, a display portion 9103 is incorporated in a housing9101. The display portion 9103 can display images, and a light-emittingdevice formed according to one embodiment of the present invention canbe used for the display portion 9103. In addition, the housing 9101 issupported by a stand 9105.

The television set 9100 can be operated with an operation switch of thehousing 9101 or a separate remote controller 9110. Channels and volumecan be controlled with an operation key 9109 of the remote controller9110 so that an image displayed on the display portion 9103 can becontrolled. Furthermore, the remote controller 9110 may be provided witha display portion 9107 for displaying data output from the remotecontroller 9110.

Note that the television set 9100 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Furthermore, when the television set 9100is connected to a communication network by wired or wireless connectionvia the modem, one-way (from a transmitter to a receiver) or two-way(between a transmitter and a receiver, between receivers, or the like)data communication can be performed.

In addition, the light-emitting device formed according to oneembodiment of the present invention has lower power consumption. Thus,the light-emitting device is used for the display portion 9103 of thetelevision set and thereby a long-lifetime television set can beprovided.

FIG. 9B illustrates a computer, which includes a main body 9201, achassis 9202, a display portion 9203, a keyboard 9204, an externalconnection port 9205, a pointing device 9206, and the like. Thiscomputer is manufactured using a light-emitting device according to oneembodiment of the present invention for the display portion 9203.

In addition, the light-emitting device formed according to oneembodiment of the present invention has lower power consumption. Thus,the light-emitting device is used for the display portion 9203 of thecomputer and thereby a long-lifetime computer can be provided.

FIG. 9C illustrates a portable game machine including two housings, ahousing 9301 and a housing 9302 which are jointed with a connector 9303so as to be openable and closable. A display portion 9304 isincorporated in the housing 9301, and a display portion 9305 isincorporated in the housing 9302. In addition, the portable game machineillustrated in FIG. 9C is provided with a speaker portion 9306, arecording medium insert portion 9307, an LED lamp 9308, input means(operation keys 9309, a connection terminal 9310, a sensor 9311 (havinga function of measuring force, displacement, position, speed,acceleration, angular velocity, rotation number, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radial ray,flow rate, humidity, gradient, vibration, smell, or infrared ray), and amicrophone 9312), and the like. It is needless to say that the structureof the portable amusement machine is not limited to the above and otherstructures in which a light-emitting device formed according to oneembodiment of the present invention is used for at least one or both ofthe display portion 9304 and the display portion 9305 may be employed.The portable amusement machine may include other accessory equipment asappropriate. The portable amusement machine illustrated in FIG. 9C has afunction of reading a program or data stored in a recording medium todisplay it on the display portion, and a function of sharing informationwith another portable amusement machine by wireless communication. Theportable game machine in FIG. 9C can have various functions withoutbeing limited to this example.

In addition, the light-emitting device formed according to oneembodiment of the present invention has lower power consumption. Thus,the light-emitting device is used for the display portions (9304, 9305)of the portable amusement machine and thereby a long-lifetime portableamusement machine can be provided.

FIG. 9D illustrates an example of a mobile phone. The mobile phone 9400is provided with a display portion 9402 incorporated in a housing 9401,operation buttons 9403, an external connection port 9404, a speaker9405, a microphone 9406, and the like. Note that the mobile phone 9400is manufactured using a light-emitting device formed according to oneembodiment of the present invention for the display portion 9402.

When the display portion 9402 of the mobile phone 9400 illustrated inFIG. 9D is touched with a finger or the like, data can be input into themobile phone 9400. Users can make a call or text messaging by touchingthe display portion 9402 with their fingers or the like.

There are mainly three screen modes of the display portion 9402. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting information such as text. Thethird mode is a display-and-input mode in which two modes of the displaymode and the input mode are mixed.

For example, in making a call or text messaging, the display portion9402 is set to a text input mode where text input is mainly performed,and text input operation can be done on a screen. In this case, it ispreferable to display a keyboard or number buttons on almost the entirescreen of the display portion 9402.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone 9400, display on the screen of the display portion 9402 canbe automatically switched by determining the direction of the mobilephone 9400 (whether the mobile phone is placed horizontally orvertically, e.g., a landscape mode or a portrait mode).

The screen modes are changed by touching the display portion 9402 orusing the operation buttons 9403 of the housing 9401. Alternatively, thescreen modes can be switched depending on kinds of images displayed inthe display portion 9402. For example, when a signal for an imagedisplayed in the display portion is data of moving images, the screenmode is switched to the display mode. When the signal is text data, thescreen mode is switched to the input mode.

Furthermore, in the input mode, when input by touching the displayportion 9402 is not performed for a certain period while a signal isdetected by the optical sensor in the display portion 9402, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 9402 can also function as an image sensor. Forexample, an image of a palm print, a fingerprint, or the like is takenby touching the display portion 9402 with the palm or the finger,whereby personal authentication can be performed. Furthermore, byproviding a backlight or a sensing light source emitting a near-infraredlight for the display portion, an image of a finger vein, a palm vein,or the like can also be taken.

In addition, the light-emitting device formed according to oneembodiment of the present invention has lower power consumption. Thus,the light-emitting device is used for the display portion 9402 of themobile phone 9400 and thereby a long-lifetime mobile phone can beprovided.

FIG. 9E illustrates a lighting device (a desk lamp) including a lightingportion 9501, a shade 9502, an adjustable arm 9503, a support 9504, abase 9505, and a power supply switch 9506. This lighting device ismanufactured using a light-emitting device according to one embodimentof the present invention for the lighting portion 9501. Note that theterm “lighting device” also encompasses ceiling lights (ceiling-fixedlighting devices), wall lights (wall-hanging lighting devices), and thelike, as well as the desk lamp illustrated in FIG. 9E.

In addition, the light-emitting device formed according to oneembodiment of the present invention has lower power consumption. Thus,the light-emitting device is used for the lighting portion 9501 of thelighting device (desk lamp) and thereby a long-lifetime lighting device(desk lamp) can be provided.

FIG. 10 illustrates an example in which a light-emitting device to whichone embodiment of the present invention is applied is used as aninterior lighting device. Since the light-emitting device of oneembodiment of the present invention can also have a larger area, it canbe used as a lighting device having a large area as illustrated by aceiling light 1001. Additionally, the light-emitting device can be usedas a wall light 1002. Since the light-emitting device formed accordingto one embodiment of the present invention has the light-emittingelement with low driving voltage, it can be used as the lighting devicewith low power consumption. As illustrated in FIG. 10, a desk lamp 1003illustrated in FIG. 9E may be used together in the room provided withthe interior lighting device.

In the above-described manner, electronic devices or lighting devicescan be obtained by application of a light-emitting device which is oneembodiment of the present invention. As described above, the applicablerange of the light-emitting device of one embodiment of the presentinvention is so wide that the light-emitting device can be applied toelectronic devices in a variety of fields.

Note that the structure described in this embodiment can be combinedwith any of the structures shown in Embodiments 1 to 6 as appropriate.

EXAMPLES Example 1

In Example 1, a light-emitting element according to one embodiment ofthe present invention will be described. Structural formulae ofmaterials used in this example are shown below.

(Fabrication of Light-Emitting Element A)

First, indium tin oxide containing silicon or silicon oxide wasdeposited to a thickness of 110 nm over a glass substrate by asputtering method to form an anode (an electrode area: 2 mm×2 mm).

Next, the glass substrate provided with the anode was fixed to asubstrate holder provided in a vacuum evaporation apparatus such thatthe side on which the anode was formed faced downward. The vacuumevaporation apparatus was evacuated to approximately 10⁻⁴ Pa, and then,a first charge production region was formed by co-evaporation of4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (abbreviation: NPB) as ahigh hole-transporting substance and molybdenum(VI) oxide as an acceptorsubstance. The film thickness of the first charge production region was50 nm, and the weight ratio between NPB and molybdenum(VI) oxide was setto 4:1 (=NPB:molybdenum oxide). Note that the co-evaporation method isan evaporation method in which evaporation is carried out from aplurality of evaporation sources at the same time in one treatmentchamber.

Next, NPB was formed to a thickness of 10 nm by an evaporation methodusing resistance heating to form a hole-transporting layer.

Next, 9-[4-(10-phenyl-9-antryl)phenyl]-9H-carbazole (abbreviation: CzPA)and N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(2PCAPA) were co-evaporated with the weight ratio of CzPA:2PCAPA=1:0.05to form the light-emitting layer. CzPA is an electron-transportingsubstance and 2PCAPA is a substance which emits green light. Thethickness of the light-emitting layer was 30 nm.

After that, by an evaporation method using resistance heating,tris(8-quinolinolato)aluminum (Alq) was deposited to a thickness of 10nm, and then bathophenanthroline (BPhen) was deposited to a thickness of10 nm to form an electron-transporting layer.

Then, lithium oxide (Li₂O) was deposited to a thickness of about 0.1 nmto form an electron-injecting buffer by an evaporation method usingresistance heating similarly, and then3,4,9,10-perylenetetracarboxylicbisbenzimidazole (abbreviation: PTCBI)was deposited to a thickness of about 3 nm to form an electron-relaylayer by an evaporation method using resistance heating similarly.

Then, 4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (abbreviation: NPB)as a high hole-transporting substance and molybdenum(VI) oxide as anacceptor substance were co-evaporated to form a second charge productionregion. The thickness was 20 nm, and the weight ratio between NPB andmolybdenum(VI) oxide was set 4:1 (=NPB:molybdenum oxide).

Next, a 200-nm-thick aluminum film was deposited to form a cathode, sothat the light-emitting element A was fabricated.

(Fabrication of a Reference Light-Emitting Element a-1)

Steps up to formation of an electron-injecting buffer were conducted ina manner similar to the light-emitting element A. Then, a cathode wasformed without formation of an electron-relay layer and a second chargeproduction region, and the thus formed light-emitting element wasregarded as the reference light-emitting element a-1.

(Fabrication of a Reference Light-Emitting Element a-2)

Steps up to formation of an electron-injecting buffer layer wereconducted in a manner similar to the light-emitting element A. Then, asecond charge production region and a cathode were formed withoutformation of an electron-relay layer, and the thus formed light-emittingelement was regarded as the reference light-emitting element a-2.

Table 1 below shows structural parts of the light-emitting element A,the reference light-emitting element a-1, and the referencelight-emitting element a-2. Note that the light-emitting elements allhave the same structures of anode, first charge production region,hole-transporting layer, light-emitting layer, and electron-transportinglayer, and description of the first charge production region,hole-transporting layer, light-emitting layer and electron-transportinglayer is omitted.

TABLE 1 electron-injecting electron-relay second charge anode * bufferlayer layer production region cathode A NITO ** Li₂O PTCBI(3 nm)NPB:MoO_(x)(20 nm 4:1) Al a-1 (110 nm) (0.1 nm) — — (200 nm) a-2 —NPB:MoO_(x)(20 nm 4:1) * first charge production region,hole-transporting layer, light-emitting layer, and electron-transportinglayer are omitted ** description of material and thickness are omitted

The thus formed light-emitting element A, reference light-emittingelement a-1, and reference light-emitting element a-2 were sealed sothat the light-emitting elements were not exposed to atmospheric air ina glove box under a nitrogen atmosphere. Then, the operatingcharacteristics of the light-emitting elements were measured. Themeasurement was carried out at room temperature (under an atmospherekept at 25° C.).

FIG. 11 and FIG. 12 show voltage-luminance characteristics andvoltage-current density characteristics of the light-emitting element A,reference light-emitting element a-1, and reference light-emittingelement a-2, respectively. Table 2 shows initial values of maincharacteristics of the light-emitting elements at around 1000 cd/m².

TABLE 2 chromaticity current efficiency external quantum voltage (V) (x,y) (cd/A) efficiency (%) A 3.4 (0.29, 0.61) 14 4.1 a-1 3.4 (0.28, 0.62)14 4.2 a-2 3.9 (0.29, 0.61) 15 4.4

As apparent from FIG. 11, the light-emitting element A has a differentstructure from the reference light-emitting element a-1 and an increasedthickness between electrodes, but can have substantially the sameluminance with respect to the same voltage, as the referencelight-emitting element a-1. In addition, as comparison with thereference light-emitting element a-2, high luminance with respect to thesame voltage can be obtained by provision of the electron-relay layer.Note that green emission at a wavelength of around 520 nm from 2PCAPA asthe light-emitting substance of all the light-emitting elements can beobtained.

In addition, also from the voltage-current density characteristics shownin FIG. 12, it is apparent that the light-emitting element A hassubstantially the same current density as the reference light-emittingelement a-1. Additionally, current efficiencies of the light-emittingelement A, the reference light-emitting element a-1, and the referencelight-emitting element a-2 are about 14 cd/A at 1000 cd/m², which aresubstantially the same for the light-emitting elements.

From the above-described results, even when the thickness of the layerincluding an organic compound provided between the electrodes of thelight-emitting element is changed, the increase of driving voltage canbe suppressed.

Example 2

In Example 2, a light-emitting element of one embodiment of the presentinvention will be described. Structural formulae of materials used inExample 2 are referred to those in Example 1, and description thereof isomitted in Example 2.

(Fabrication of a Light-Emitting Element B)

After an anode, a first charge production region, a hole-transportinglayer, and a light-emitting layer are formed using materials similar toand by a method similar to those of the light-emitting element Adescribed in Example 1, tris(8-quinolinolato)aluminum (abbreviation:Alq) was deposited to a thickness of 10 nm to form anelectron-transporting layer by an evaporation method using resistanceheating.

Next, by an evaporation method using resistance heating,bathophenanthroline (abbreviation: BPhen) and lithium (Li) wereco-evaporated to form an electron-injecting buffer. The thickness was 10nm and the weight ratio of BPhen to lithium was adjusted to 1:0.02(=BPhen:lithium).

Then, similar to the light-emitting element A, an electron-relay layer,a second charge production region and a cathode were sequentially formedto fabricate the light-emitting element B.

(Fabrication of a Reference Light-Emitting Element b-1)

Steps up to formation of an electron-injecting buffer were conducted ina manner similar to the light-emitting element B. Then, a cathode wasformed without formation of an electron-relay layer and a second chargeproduction region, and the thus formed light-emitting element wasregarded as the reference light-emitting element b-1.

(Fabrication of a Reference Light-Emitting Element b-2)

Steps up to formation of an electron-injecting buffer layer wereconducted in a manner similar to the light-emitting element B. Then, asecond charge production region and a cathode were formed withoutformation of an electron-relay layer, and the thus formed light-emittingelement was regarded as the reference light-emitting element b-2.

Table 3 below shows structural parts of the light-emitting element B,the reference light-emitting element b-1, and the referencelight-emitting element b-2. Note that the light-emitting elements allhave the same structures of anode, first charge production region,hole-transporting layer, light-emitting layer, and electron-transportinglayer, and thus description of the first charge production region,hole-transporting layer, light-emitting layer and electron-transportinglayer is omitted.

TABLE 3 electron-injecting electron-relay second charge anode * bufferlayer layer production region cathode B NITO ** Bphen:Li PTCBI(3 nm)NPB:MoO_(x)(20 nm 4:1) Al b-1 (110 nm) (10 nm 1:0.02) — — (200 nm) b-2 —NPB:MoO_(x)(20 nm 4:1) * first charge production region,hole-transporting layer, light-emitting layer, and electron-transportinglayer are omitted ** description of material and thickness are omitted

The thus formed light-emitting element B, reference light-emittingelement b-1, and reference light-emitting element b-2 were sealed sothat the light-emitting elements were not exposed to atmospheric air ina glove box under a nitrogen atmosphere. Then, the operatingcharacteristics of the light-emitting elements were measured. Themeasurement was carried out at room temperature (under an atmospherekept at 25° C.).

FIG. 13 and FIG. 14 show voltage-luminance characteristics andvoltage-current density characteristics of the light-emitting element B,reference light-emitting element b-1, and reference light-emittingelement b-2, respectively. Table 4 shows initial values of maincharacteristics of the light-emitting elements at around 1000 cd/m².

TABLE 4 chromaticity current efficiency external quantum voltage (V) (x,y) (cd/A) efficiency (%) B 3.6 (0.29, 0.61) 13 3.9 b-1 3.7 (0.28, 0.62)14 4.0 b-2 4.0 (0.29, 0.61) 14 4.1

As apparent from FIG. 13, the light-emitting element B has a differentstructure from the reference light-emitting element b-1 and an increasedthickness between electrodes, but can have substantially the sameluminance with respect to voltage, as the reference light-emittingelement b-1. In addition, as comparison with the referencelight-emitting element b-2, high luminance with respect to voltage canbe obtained by provision of the electron-relay layer. Note that greenemission at a wavelength of around 520 nm from 2PCAPA as thelight-emitting substance of all the light-emitting elements can beobtained.

In addition, also from the voltage-current density characteristics shownin FIG. 14, it is apparent that the light-emitting element B hassubstantially the same current density as the reference light-emittingelement b-1. Additionally, current efficiencies of the light-emittingelement B, the reference light-emitting element b-1, and the referencelight-emitting element b-2 are about 14 cd/A at 1000 cd/m², which aresubstantially the same for the light-emitting elements.

From the above-described results, even when the thickness of the layerincluding an organic compound provided between the electrodes of thelight-emitting element is changed, the increase of driving voltage canbe suppressed.

Example 3

In Example 3, a light-emitting element of one embodiment of the presentinvention will be described. Structural formulae of materials used inExample 3 are referred to those in Example 1, and description thereof isomitted in Example 3.

(Fabrication of a Light-Emitting Element C)

After an anode, a first charge production region, a hole-transportinglayer, a light-emitting layer, an electron-transporting layer, anelectron-injecting buffer, and an electron relay layer were formed usingmaterials similar to and by a method similar to those of thelight-emitting element A described in Example 1,tris(8-quinolinolato)aluminum (abbreviation: Alq) was deposited to athickness of 10 nm by an evaporation method using resistance heating andsimilarly bathophenanthroline (abbreviation: BPhen) was deposited to athickness of 10 nm by an evaporation method using resistance heating toform an electron-transporting layer.

Next, by an evaporation method using resistance heating, molybdenum(VI)oxide as an acceptor substance was deposited to a thickness of 10 nm,and then, 4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (abbreviation:NPB) which was a high hole-transporting substance by an evaporationmethod using resistance heating was similarly deposited to a thicknessof 10 nm to form a second charge production region.

Then, a 200-nm-thick aluminum film was deposited to form a cathode, sothat the light-emitting element C was fabricated.

(Fabrication of a Reference Light-Emitting Element c-1)

Steps up to formation of an electron-injecting buffer were conducted ina manner similar to the light-emitting element C. Then, a second chargeproduction region and a cathode were formed without formation of anelectron-relay layer, and the thus formed light-emitting element wasregarded as the reference light-emitting element c-1.

Table 5 below shows structural parts of the light-emitting element C,and the reference light-emitting element c-1. Note that thelight-emitting elements both have the same structure of anode, firstcharge production region, hole-transporting layer, light-emitting layer,electron-transporting layer, and electron-injecting buffer anddescription of the first charge production region, hole-transportinglayer, light-emitting layer and electron-transporting layer is omitted.

TABLE 5 electron-injecting electron-relay second charge anode * bufferlayer layer production region cathode C NITO ** Li₂O PTCBI(3 nm)MoO_(x)(10 nm) NPB(10 nm) Al c-1 (110 nm) (0.1 nm) — MoO_(x)(10 nm)NPB(10 nm) (200 nm) * first charge production region, hole-transportinglayer, light-emitting layer, and electron-transporting layer are omitted** description of material and thickness are omitted

The thus formed light-emitting element C, and reference light-emittingelement c-1 were sealed so that the light-emitting elements were notexposed to atmospheric air in a glove box under a nitrogen atmosphere.Then, the operating characteristics of the light-emitting elements weremeasured. The measurement was carried out at room temperature (under anatmosphere kept at 25° C.).

FIG. 15 and FIG. 16 show voltage-luminance characteristics andvoltage-current density characteristics of the light-emitting element Cand reference light-emitting element c-1, respectively. Table 6 showsinitial values of main characteristics of the light-emitting elements ataround 1000 cd/m².

TABLE 6 chromaticity current efficiency external quantum voltage (V) (x,y) (cd/A) efficiency (%) C 4.8 (0.29, 0.61) 14 4.1 c-1 6.6 (0.29, 0.62)15 4.3

As apparent from FIG. 15, the light-emitting element C has higherluminance than the reference light-emitting element c-1 with respect tothe same voltage. This is thought to be as a result from that carriertransfer can be conducted smoothly inside the light-emitting element byprovision of the electron-relay layer. Note that green emission at awavelength of around 520 nm from 2PCAPA as the light-emitting substanceof both the light-emitting elements can be obtained.

In addition, also from the voltage-current density characteristics shownin FIG. 16, it is apparent that the light-emitting element C has highercurrent density than the reference light-emitting element c-1.Additionally, current efficiencies of the light-emitting element C andthe reference light-emitting element c-1 are about 14 cd/A at 1000cd/m², which are substantially the same for the light-emitting elements.

From the above-described results, by provision of the electron-relaylayer, electron injection into the EL layer of the light-emittingelement can be facilitated, and thus the increase of driving voltage canbe suppressed.

Example 4

In Example 4, a light-emitting element of one embodiment of the presentinvention will be described. Structural formulae of materials used inExample 4 are referred to those in Example 1, and description thereof isomitted in Example 4.

(Fabrication of a Light-Emitting Element D)

After an anode, a first charge production region, a hole-transportinglayer, a light-emitting layer, an electron-transporting layer, anelectron-injecting buffer, and an electron relay layer were formed usingmaterials similar to and by a method similar to those of thelight-emitting element B described in Example 2, molybdenum(VI) oxidewhich was an acceptor substance was deposited to a thickness of 10 nm byan evaporation method using resistance heating and4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (abbreviation: NPB) whichwas a high hole-transporting substance by an evaporation method usingresistance heating was similarly deposited to a thickness of 10 nm toform a second charge production region.

Then, a 200-nm-thick aluminum film was deposited to form a cathode, sothat the light-emitting element D was fabricated.

(Fabrication of a Reference Light-Emitting Element d-1)

Steps up to formation of an electron-injecting buffer were conducted ina manner similar to the light-emitting element D. Then, a second chargeproduction region and a cathode were formed without formation of anelectron-relay layer, and the thus formed light-emitting element wasregarded as the reference light-emitting element d-1.

Table 7 below shows structural parts of the light-emitting element D andthe reference light-emitting element d-1. Note that the light-emittingelements both have the same structure of anode, first charge productionregion, hole-transporting layer, light-emitting layer,electron-transporting layer, and electron-injecting buffer anddescription of the first charge production region, hole-transportinglayer, light-emitting layer and electron-transporting layer is omitted.

TABLE 7 electron-injecting electron-relay second charge anode * bufferlayer layer production region cathode D NITO ** Bphen:Li PTCBI(3 nm)MoO_(x)(10 nm) NPB(10 nm) Al d-1 (110 nm) (10 nm 1:0.02) — MoO_(x)(10nm) NPB(10 nm) (200 nm) * first charge production region,hole-transporting layer, light-emitting layer, and electron-transportinglayer are omitted ** description of material and thickness are omitted

The thus formed light-emitting element D and reference light-emittingelement d-1 were sealed so that the light-emitting elements were notexposed to atmospheric air in a glove box under a nitrogen atmosphere.Then, the operating characteristics of the light-emitting elements weremeasured. The measurement was carried out at room temperature (under anatmosphere kept at 25° C.).

FIG. 17 and FIG. 18 show voltage-luminance characteristics andvoltage-current density characteristics of the light-emitting element Dand reference light-emitting element d-1, respectively. Table 8 showsinitial values of main characteristics of the light-emitting elements ataround 1000 cd/m².

TABLE 8 chromaticity current efficiency external quantum voltage (V) (x,y) (cd/A) efficiency (%) D 5.0 (0.29, 0.61) 13 3.9 d-1 6.0 (0.29, 0.61)13 4.1

As apparent from FIG. 17, the light-emitting element D has higherluminance than the reference light-emitting element d-1 with respect tothe same voltage. This is thought to be as a result from that carriertransfer can be conducted smoothly inside the light-emitting element byprovision of the electron-relay layer. Note that green emission at awavelength of around 520 nm from 2PCAPA as the light-emitting substanceof both the light-emitting elements can be obtained.

In addition, also from the voltage-current density characteristics shownin FIG. 18, it is apparent that the light-emitting element D has highercurrent density than the reference light-emitting element d-1.Additionally, current efficiencies of the light-emitting element D andthe reference light-emitting element d-1 are about 13 cd/A at 1000cd/m², which are substantially the same for the light-emitting elements.

From the above-described results, by provision of the electron-relaylayer, electron injection into the EL layer of the light-emittingelement can be facilitated and thus the increase of driving voltage canbe suppressed.

This application is based on Japanese Patent Application Serial No.2008-306153 filed with Japan Patent Office on Dec. 1, 2008, and JapanesePatent Application Serial No. 2009-130539 filed with Japan Patent Officeon May 29, 2009, the entire contents of which are hereby incorporated byreference.

1. A light-emitting element comprising: an EL layer including alight-emitting layer between an anode and a cathode; a first layerincluding a first hole-transporting substance and a first acceptorsubstance, provided in direct contact with the cathode between thecathode and the EL layer; a second layer including a firstelectron-transporting substance, provided in direct contact with thefirst layer; and a third layer including a second electron-transportingsubstance and an element, provided in direct contact with the secondlayer and the EL layer.
 2. The light-emitting element according to claim1, wherein the element is a donor substance.
 3. The light-emittingelement according to claim 1, wherein the element is at least oneselected from the group consisting of an alkali metal, an alkaline earthmetal, a rare earth metal, an alkali metal compound, an alkaline earthmetal compound, and a rare earth metal compound.
 4. The light-emittingelement according to claim 1, wherein a mass ratio of the element to thesecond electron-transporting substance is from 0.001:1 to 0.1:1.
 5. Thelight-emitting element according to claim 1, wherein the EL layerincludes a fourth layer including a third electron-transportingsubstance, and wherein the fourth layer is in direct contact with thethird layer.
 6. The light-emitting element according to claim 1, whereina mass ratio of the first acceptor substance to the firsthole-transporting substance is from 0.1:1 to 4.0:1.
 7. Thelight-emitting element according to claim 1, wherein the firstelectron-transporting substance has a LUMO level of −5.0 eV or higher.8. The light-emitting element according to claim 1, wherein the firstelectron-transporting substance is a perylene derivative ornitrogen-containing condensed aromatic compound.
 9. The light-emittingelement according to claim 1, wherein the first acceptor substance is atransition metal oxide or an oxide of a metal belonging to Groups 4 to 8in a periodic table.
 10. The light-emitting element according to claim1, wherein the first acceptor substance is molybdenum oxide.
 11. Alight-emitting element comprising: an EL layer including alight-emitting layer between an anode and a cathode; a first layerincluding a first hole-transporting substance and a first acceptorsubstance, provided in direct contact with the cathode between thecathode and the EL layer; a second layer including a firstelectron-transporting substance, provided in direct contact with thefirst layer; and a third layer including a second electron-transportingsubstance and an element, provided in direct contact with the secondlayer and the EL layer, wherein the first layer has a stacked structureof a layer including the first hole-transporting substance and a layerincluding the first acceptor substance.
 12. The light-emitting elementaccording to claim 11, wherein the element is a donor substance.
 13. Thelight-emitting element according to claim 11, wherein the element is atleast one selected from the group consisting of an alkali metal, analkaline earth metal, a rare earth metal, an alkali metal compound, analkaline earth metal compound, and a rare earth metal compound.
 14. Thelight-emitting element according to claim 11, wherein the EL layerincludes a fourth layer including a third electron-transportingsubstance, and wherein the fourth layer is in direct contact with thethird layer.
 15. The light-emitting element according to claim 11,wherein the first electron-transporting substance has a LUMO level of−5.0 eV or higher.
 16. The light-emitting element according to claim 11,wherein the first electron-transporting substance is a perylenederivative or nitrogen-containing condensed aromatic compound.
 17. Thelight-emitting element according to claim 11, wherein the first acceptorsubstance is a transition metal oxide or an oxide of a metal belongingto Groups 4 to 8 in a periodic table.
 18. A light-emitting elementcomprising: an EL layer including a light-emitting layer between ananode and a cathode; a first layer including a first hole-transportingsubstance and a first acceptor substance, provided in direct contactwith the cathode between the cathode and the EL layer; a second layerincluding a first electron-transporting substance, provided in directcontact with the first layer; and a third layer including a secondelectron-transporting substance and an element, provided in directcontact with the second layer and the EL layer, wherein the EL layerincludes a fifth layer including a second hole-transporting substanceand a second acceptor substance, and wherein the fifth layer is indirect contact with the anode.
 19. The light-emitting element accordingto claim 18, wherein a mass ratio of the second acceptor substance tothe second hole-transporting substance is from 0.1:1 to 4.0:1.
 20. Thelight-emitting element according to claim 18, wherein the fifth layerhas a stacked structure of a layer including the secondhole-transporting substance and a layer including the second acceptorsubstance.
 21. The light-emitting element according to claim 18, whereinthe element is a donor substance.
 22. The light-emitting elementaccording to claim 18, wherein the element is at least one selected fromthe group consisting of an alkali metal, an alkaline earth metal, a rareearth metal, an alkali metal compound, an alkaline earth metal compound,and a rare earth metal compound.
 23. The light-emitting elementaccording to claim 18, wherein the EL layer includes a fourth layerincluding a third electron-transporting substance, and wherein thefourth layer is in direct contact with the third layer.
 24. Thelight-emitting element according to claim 18, wherein the firstelectron-transporting substance has a LUMO level of −5.0 eV or higher.25. The light-emitting element according to claim 18, wherein the firstelectron-transporting substance is a perylene derivative ornitrogen-containing condensed aromatic compound.
 26. The light-emittingelement according to claim 18, wherein the first acceptor substance is atransition metal oxide or an oxide of a metal belonging to Groups 4 to 8in a periodic table.
 27. A light-emitting device, which uses thelight-emitting element according to any one of claims 1, 11, and
 18. 28.An electronic device, which uses the light-emitting device according toclaim
 27. 29. A lighting device, which uses the light-emitting deviceaccording to claim 27.